1. Field of the Invention
This invention relates generally to the purificaion of impure solutions of trivalent thallium compounds, and more specifically to the treatment of carboxylic acid media containing dissolved trivalent thallium carboxylates and monovalent thallium carboxylates.
2. Description of the Prior Art
Trivalent thallium compounds, i.e., thallic compounds, have been used as oxidizing agents in various reactions. For example, Kruse et al. J. Org. Chem. 36, 1154 (1971) describes the epoxidation of certain olefins with thallic acetate, and U.S. Pat. No. 3,641,067 (issued in 1972 to W. Kruse) relates to the preparation of the epoxides of propylene and isobutylene by means of lower thallic alkanoates.
In all of these reactions the trivalent thallium is reduced to the monovalent state and if the thallium is to be reused in the reaction it is necessary to reoxidize or "regenerate" it by converting thallium (I) to thallium (III). Various methods for effecting this conversion have been proposed. Thus, it is proposed to convert thallium (I) to thallium (III) in the application of William Brill, entitled "Catalytic Conversion of Thallium (I) to Thallium (III)", Ser. No. 789,053, filed Apr. 21, 1977 now U.S. Pat. No. 4,115,420 by means of molecular oxygen using a Group VIII noble metal as a catalyst. In addition, it has been proposed in the application of Richard A. Johnson entitled, "Conversion of Mono-valent Thallium to Tri-valent Thallium," Ser. No. 740,147, filed Nov. 8, 1976 now U.S. Pat. No. 4,113,756 to convert thallium (I) to thallium (III) by means of molecular oxygen in the presence of a Group VIII noble metal catalyst and in the presence of a promoter comprising an alkali metal compound. The processes of both Ser. No. 789,053 and Ser. No. 740,147 typically result in an aqueous solution containing the desired trivalent thallium compound together with unconverted monovalent thallium compound and alkali metal compound, where the latter is employed as promoter in accordance with Ser. No. 740,147. It is generally desirable to recover the trivalent thallium compound so produced to avoid passing monovalent thallium compound and alkali metal compound (where present) as impurities to the subsequent reaction steps using the trivalent thallium compound.
In an application of Richard A. Johnson, entitled "Extractive Recovery of Trivalent Thallium Values from Aqueous Solutions," Ser. No. 896,550 said application being filed on even date herewith (and which is hereby incorporated by reference), a method of removing trivalent thallium values from aqueous solutions containing trivalent thallium values, monovalent thallium values, and optionally, alkali metal compound, is disclosed in which the aqueous solution is contacted with an extractant comprising a carboxylic acid having limited solubility with water to preferentially extract at least a portion of the dissolved trivalent thallium values into the carboxylic acid. A carboxylic acid extract containing the extracted trivalent thallium values is then recovered as a separate phase. While such a process efficiently removes trivalent thallium values from aqueous solutions of the same, monovalent thallium and alkali metal values (where present) can also be extracted to some extent by the carboxylic acid extractant. Removal of residual monovalent thallium and alkali metal values from the carboxylic acid extract is therefore desirable to further isolate the trivalent thallium values. Treatment of the carboxylic acid extract for precipitation of monovalent thallium as the halide and use of ion-exchange resins to remove alkali metal values as suggested in the above application of R. Johnson are effective, but an alternative method would be desirable. While the water back-extraction suggested in the above application of R. Johnson results in the removal of a portion of the undesired monovalent thallium and alkali metal values, a need exists for a more efficient extraction process.
In Japanese Patent Application application 50-92296 (published July 23, 1975), trivalent thallium ions are extracted from aqueous solutions containing the same by using a polar organic solvent in the presence of bromine and/or chlorine ions and hydrogen bromide and/or hydrogen chloride to form an extractant containing the corresponding halogen thallium acid (e.g., HTlBr.sub.4, HTlCl.sub.4, etc.), and the extractant so produced is simultaneously contacted with a small amount of water and a water-immiscible non-polar organic solvent, whereby the trivalent thallium ions are reextracted into the aqueous phase as the halogen thallium acid. This process is disadvantageous for treatment of aqueous solutions containing both monovalent and trivalent thallium values in that any monovalent thallium ions will be precipitated as the corresponding halide, which precipitate must then be recovered and treated for conversion of the monovalent thallium content thereof to a water-soluble form before further processing can be effected, e.g., before recycle of the monovalent thallium values to an oxidation step in which trivalent thallium values are formed therefrom.
Other extraction techniques have been developed as analytical tools for recovery of trivalent thallium ions from aqueous solutions, employing such extractants as diethyldithiocarbamate and 8-quinolinol-4-thenoyltrifluoroacetone, as outlined in G. H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry" 237 (John Wiley & Sons 1957). However, such analytical extraction methods are not readily adaptable to industrial processes and offer the disadvantage of the extreme high cost of such extractants on an industrial scale.
Likewise, recovery of trivalent thallium ions in the form of TlCl.sub.4.sup.- using an anion exchange resin as suggested in U.S. Pat. No. 3,399,956 (issued in 1968 to I. Hirose et al.) is disadvantageous due to the high cost of the resin required for recovery of large amounts of trivalent thallium in an industrial scale process.
Other extraction methods employed for treatment of liquids containing monovalent thallium are not readily adaptable to separation of trivalent thallium from aqueous mixtures containing monovalent thallium. See U.S. Pat. No. 4,031,196 (issued in 1977 to J. J. Leonard)(extraction of isobutyric acid with dibutyl ether from aqueous medium also containing barium salt and thallous isobutyrate) and A. Letheridge et al., J. Chem. Soc. Perkins I, p. 2763 (1973) (extraction of aqueous mixture containing thallous trifluoroacetate and octene-1 oxidation products with ether). However, the aqueous media treated in these references, did not contain trivalent thallium.